1. Field of the Invention
This invention relates to emulsion polymers and a process for making the same. More particularly, the invention relates to a supported emulsion polymer comprised of a graft copolymer of aromatic support resin polymer and a vinyl chloride (co)polymer.
2. Description of the Prior Art
Supported (or fortified) styrene-acrylic emulsions are commonly used in paints, stains, floor polishes, leather treatment, cement formulations, architectural and paper coatings, and graphic arts. The latter includes printing inks and coatings for printing. A supported emulsion is the one in which an aqueous resin solution (the support) is used as a medium in which the second polymer is dispersed (mechanically or made in situ via polymerization to form an emulsion). The support resin adds stability to the emulsion, including both shear and freeze-thaw stability, keeps the emulsion particle size small, which is important for gloss and clarity, and aids the flow characteristics of the composition, including its leveling and redispersibility. Supported polymer emulsions are desirable in coatings and graphic arts due to the ease of use provided by the incorporation of solution resin into the polymer. Reference can be had to the following patent documents: M. Sunada et al., U.S. Pat. No. 4,179,417; W. J. Blank et al, U.S. Pat. No. 4,151,143; R. M. Boyack et al., U.S. Pat. No. 4,385,152; L. W. Lee et al., EP No. 0 338 486 A2 ; S.-L. Tsaur, U.S. Pat. No. 4,820,762 ; R. A. Kiehlbauch et al, U.S. Pat. No. 4,839,413; G. R. Frazee, U.S. Pat. No. 4,879,333; A. B. Brown et al., U.S. Pat. No. 4,916,171; R. Kuropka, EP No. 0 627 450 A1; and S. J. Chiou et al, EP No. 0 727 451 A2.
Typical prior art support resins are made from acrylic acid, methacrylic acid, maleic acid, styrene, alpha-methylstyrene, and various acrylate monomers. An emulsion polymer then is formed in the presence of the support resin by polymerizing acrylates, styrene, alpha-methylstyrene, and other like monomers. Further, di- and polyfunctional compounds often are used to chemically bind a support resin polymer to the particles of emulsion polymer. Examples of polyfunctional compounds include allyl-, methallyl-, vinyl-, and crotyl-esters of acrylic, methacrylic, maleic, fumaric and itaconic acids, or analogous amides, or analogous mercaptans. See, for example, European Patent Nos. EP 0 348 565 A1 or EP 0 522 791 A1; or U.S Pat. Nos. 4,565,839, 4,465,803, or 4,876,313.
Styrene and alpha-methylstyrene monomers are used in support resins because they impart gloss, flow and leveling, hardness and hydrophobicity to polymeric films, which are desirable attributes when making coatings and inks for paper, packaging, and graphic arts. Polystyrene has a glass transition temperature (T.sub.g) of approximately 105.degree. C. and is responsible for imparting gloss and hydrophobicity to certain graphic arts applications. In addition, as compared to acrylics, styrene creates a hard polymer. Alpha-methylstyrene is used for the same reasons as styrene, plus it provides for molecular weight control due to its low polymerization ceiling temperature.
It would be desirable, however, to synthesize a supported emulsion of a vinyl chloride (co)polymer for use in the graphic arts industry. Poly(vinyl chloride) has a lower T.sub.g (75 to 85.degree. C. depending on tacticity) relative to polystyrene, and its presence is known to improve rub resistance and other abrasion properties useful in graphic arts applications. Further, vinyl chloride can provide better grease resistance, may improve adhesion to polyolefins, can be made into an ink or primer to accept an extrusion coat, and can lower the overall cost of an emulsion formulation, since abrasion enhancing additives such as waxes might not be required. Thus, it would be beneficial to synthesize an emulsion polymer having the attributes of both vinyl chloride (co)polymers as well as styrenic support resins.
It is well known in the prior art that styrenic monomers are poisonous to vinyl chloride polymerizations. Polymers derived from styrene, its derivatives, such as alpha-methylstyrene, and other aromatic monomers usually contain residual aromatic monomers and styrenic or other substituted aromatic unsaturation as end groups. For example, a gas chromatography analysis of typical, commercially available support resin indicates that there is from about 100 ppm to about 1500 ppm of unreacted styrenic monomers present in the support resin. Further, both types of unsaturation (free monomers and end groups) are poisonous to vinyl chloride polymerization. For example, in as early as 1948, Chapin, Earl C., Ham, George E. and Fordyce, Reid G., J. Chem. Soc., Vol. 70, p. 538 (1948) reported that a vinyl chloride-styrene copolymerization, containing 3.2 mole % styrene, could only achieve a 3.6 wt. % conversion after 340 hours. This means that the polymerization of vinyl chloride in the presence of styrene is greatly retarded and for all practical purposes is impossible to achieve.
Thus, a need exists for an emulsion polymerization process, and product formed thereby, wherein vinyl chloride can be (co)polymerized in the presence of a styrenic polymer without poisoning of the polymerization process. Particular challenge is represented by support resins made with the use of alpha-methylstyrene because the latter has low polymerization ceiling temperature of about 0-60.degree. C. (J. Brandrup and E. H. Immergut, Polymer Handbook, 3rd Edn., p. II/316 and II/319). The low ceiling temperature makes a complete homopolymerization of alpha-methylstyrene impossible at elevated temperatures, unless some measures are taken.